Diesel power plants for locomotives and trucks include a fuel system which supplies the engine injectors with fuel at a rate which always exceeds the demands of the engine. The excess fuel, whose quantity varies from a minimum at full throttle to a maximum at idle, is returned to the fuel tank, where it is comingled with the stored fuel, and subsequently is redelivered to the engine. The excess fuel usually is heated by the engine, particularly in the case of EMD locomotives; therefore, recirculation of this fuel tends to keep the entire fuel system warm.
The transportation industry normally is supplied with No. 2 grade diesel fuel, which contains wax constituents. The temperature at which this wax crystallizes, called the cloud point, varies between 5.degree. F. and 20.degree. F., depending upon the type of crude oil and the process used to refine it. In view of its relatively high cloud point, the No. 2 grade fuel can cause serious operating difficulties in the winter. If tank temperature decreases below the cloud point, the wax crystals which form in the fuel frequently clog the filters, which are located between the pump and the injectors, and thus interrupt the supply of fuel to the engine. As a result, the engine will shut down, thereby necessitating servicing, or even major repairs if a freeze-up occurs, and causing transportation delays.
Various solutions to the wax-up problem have been proposed. One, of course, is to use a better grade of fuel, such as dewaxed No. 2 or a mixture of No. 2 and No. 1 grade fuels. This solution, however, is very expensive, intolerably so in the case of a railroad. Another solution, and one which has been used by U.S. railroads, is to employ electric heaters in the wayside storage tanks, but this too is quite expensive, and it is not a complete answer at times when ambient temperatures remain below 0.degree. F. for extended periods or when the frequency at which locomotive fueling occurs is so high that there is insufficient time for the fuel to be warmed. A more attractive solution employed by the railroad industry consists in adding to the fuel system a heat exchanger which uses engine coolant to heat the fuel flowing from the pump to the filters. If the capacity of this heat exchanger is sufficient, this solution is acceptable for many of the operating conditions encountered in the winter. However, it is not effective in cases where ambient, and consequently tank, temperature remains well below the cloud point for long periods of time. Under these extreme conditions, which were encountered in the winter of 1978-1979, the filters still can become clogged with wax when the engine is idling and little heat is made available to the heat exchanger, unless, of course, idle speed is increased and fuel is wasted. Moreover, even when the engine is running at higher speeds and generates sufficient heat to prevent plugging of the filters, wax crystals can, and commonly, do, clog the suction line leading from the tank to the inlet of the fuel pump. The reason for this will be evident when it is recalled that diesel fuel is a relatively good heat insulator. Because of this characteristic, there is a large temperature gradient through the fuel tank, and wax crystals will develop in those regions remote from the one to which the warm excess fuel is returned.
It has also been proposed to provide in the fuel tank a confined zone or well in which the warm fuel returned from the engine is combined with stored fuel to create a heated mixture which is supplied to the suction line of the fuel pump. The admission of return fuel to the well is controlled by a temperature responsive valve which senses the temperature of the output flow and serves either as a switch which directs return flow to the well or to a remote region of the tank, or as a flow divider which splits the return flow between these two destinations. A scheme of this kind may be effective to prevent detrimental waxing under idle conditions, but, since it does not insure that any particular fraction of the returning fuel is included in the output mixture, its effectiveness during full throttle operation, when the volume of the returning fuel is relatively small, is questionable. Furthermore, the scheme is considered unattractive because it requires use of a thermostatic control, which sometimes is unreliable and always requires maintenance. In fact, the poor performance record of such elements is so well known that at least one locomotive supplier recommends routine replacement of these parts every two years.